Nitrous oxide is a chemically active trace gas which is believed to contribute to the recent increase in the Earth's surface temperature by absorbing reflected infrared radiation. According to scientific studies the global warming potential of each molecule of nitrous oxide emitted is about 290 times that of the carbon dioxide molecule. Furthermore, the atmospheric lifetime of nitrous oxide molecules in the environment is estimated to be approximately 150 years. Although the impact of man-made nitrous oxide is less well-defined, it is known that increases in nitrous oxide in the atmosphere will eventually result in increased ozone destruction. Adipic acid is an important synthetic chemical used in the manufacture of a nylon polymer, nylon 6,6 polyamide, which has been identified as a source of man-made nitrous oxide in the atmosphere. This nylon polymer is used throughout the world in carpets, tire cord, apparel, upholstery, auto parts, and in many other products which impact our life every day. Typically, adipic acid is produced from cyclohexane wherein the cyclohexane is converted to mixtures of the alcohol and ketone: cyclohexanol and cyclohexanone. The alcohol and ketone mixture is subsequently oxidized with nitric acid to produce adipic acid. The nitric acid oxidation of the cyclohexanol mixture results in the production of approximately one mole of nitrous oxide per mole of adipic acid produced. Some nitrous oxide which is contained in the reaction off-gases is emitted to the atmosphere. Estimates provided in an article entitled, Abatement of Nitrous Oxide Emissions Produced in the Adipic Acid Industry, by R. A. Reimer, C. S. Slaten, M. Seapan, M. W. Lower and P. E. Tomlinson, published in AIChE's Environmental Progress, Vol. 13, No. 2, May 1994, pp. 134-137, suggest that in 1990 about 68 percent of the nitrous oxide produced as a dilute waste gas stream from the manufacturing of adipic acid was ultimately emitted to the atmosphere. The basic technology for producing adipic acid by the nitric acid oxidation of cyclohexanol, cyclohexanone, or a mixture thereof is well-known and is described in Ullman's Encyclopedia of Industrial Chemistry, 5.sup.th /Edition, Volume A1, edited by Wolfgang Gerhartz et al., (1997), pages 269-272, and in the Encyclopedia of Chemical Processing and Design, edited by John J. McKetta, published by Marcel Dekker, Inc., (19), Vol. 2, pages 128-142, herein incorporated by reference. Cyclohexane may be produced from phenol conventionally by mild hydrogenation. U.S. Pat. No. 5,110,995 discloses a process for the preparation of phenol or phenol derivatives by the oxidation of aromatic hydrocarbons such as benzene with nitrous oxide at a temperature between about 275.degree. C. and about 450.degree. C. in the presence of a zeolite catalyst. In one proposed scheme, the nitrous oxide waste stream from the adipic acid plant will be employed as the feed to the benzene-to-phenol (Chemical Marketing Reporter, Vol. 251, No. 1, Jan. 6, 1997).
Pressure swing adsorption (PSA) provides an efficient and economical means for separating a multi-component gas stream containing at least two gases having different adsorption characteristics. The more strongly adsorbable gas can be an impurity which is removed from the less strongly adsorbable gas which is taken off as product; or, the more strongly adsorbable gas can be the desired product, which is separated from the less strongly adsorbable gas. For example, it may be desired to remove carbon monoxide and light hydrocarbons from a hydrogen-containing feed stream to produce a purified (99+%) hydrogen stream for a hydrocracking or other catalytic process where these impurities could adversely affect the catalyst or the reaction. On the other hand, it may be desired to recover more strongly adsorbable gases, such as ethylene, from a feedstream to produce an ethylene-rich product.
In pressure swing adsorption, a multi-component gas is typically fed to at least one of a plurality of adsorption zones at an elevated pressure effective to adsorb at least one component, while at least one other component passes through. At a defined time, the feedstream to the adsorber is terminated and the adsorption zone is depressurized by one or more cocurrent depressurization steps wherein pressure is reduced to a defined level which permits the separated, less strongly adsorbed component or components remaining in the adsorption zone to be drawn off without significant concentration of the more strongly adsorbed components. Then, the adsorption zone is depressurized by a countercurrent depressurization step wherein the pressure on the adsorption zone is further reduced by withdrawing desorbed gas countercurrently to the direction of feedstream. Finally, the adsorption zone is purged and repressurized. The final stage of repressurization is typically with product gas and is often referred to as product repressurization. In multi-zone systems there are typically additional steps, and those noted above may be done in stages. U.S. Pat. Nos. 3,176,444 issued to Kiyonaga, 3,986,849 issued to Fuderer et al., and 3,430,418 and 3,703,068 both issued to Wagner, among others, describe multi-zone, adiabatic pressure swing adsorption systems employing both cocurrent and countercurrent depressurization, and the disclosures of these patents are incorporated by reference in their entireties. The above-mentioned patents to Fuderer et al., and Wagner are herein incorporated by reference.
Various classes of adsorbents are known to be suitable for use in PSA systems, the selection of which is dependent upon the feedstream components and other factors generally known to those skilled in the art. In general, suitable adsorbents include molecular sieves, silica gel, activated carbon, and activated alumina. For some separations, specialized adsorbents can be advantageous. One example of such a specialized adsorbent is disclosed in U.S. Pat. No. 4,775,396. U.S. Pat. No. 4,775,396 issued to Rastelli et al. discloses a PSA process for the bulk separation of CO.sub.2 from methane, e.g., landfill gas. The patent discloses that for a landfill gas purification process, CO.sub.2 can be effectively removed from gas mixtures containing CO.sub.2 using the calcium ion-exchanged form of zeolite A, but because of the strong affinity between the sorbent and adsorbate, thermal energy is required for effective desorption of the CO.sub.2. This would suggest a thermal swing adsorption process. However, for the bulk removal of CO.sub.2 from methane, the patent discloses that PSA can be effective when using faujasite type of zeolitic aluminosilicate containing at least 20 equivalent percent of at least one cation species selected from the group consisting of zinc, rare earth, hydrogen and ammonium and containing not more than 80 equivalent percent of alkali metal or alkaline earth metal cations.
In the past, others have attempted to control nitrous oxide emissions from adipic acid production by either recovering a pure nitrous product (99%) by cryogenic means or by the chemical or thermal destruction of the nitrous oxide. However, minor amounts of carbon oxides found in the vent streams from adipic acid manufacture can make cryogenic methods expensive, requiring the removal of the carbon oxide prior to separating the nitrous oxide. Processes employing thermal destruction or catalytic decomposition of the nitrous oxide are expensive and do not result in any other benefit to the production of adipic acid. Methods are sought to substantially reduce emissions of nitrous oxide from adipic acid complexes by the recovery of the nitrous oxide.